In ink-jet technology, image quality of high-resolution images can be a function of both the ink-jet ink used to produce an image and the print medium upon which the image is printed. In ink-jet printing, droplets of ink are ejected from a printhead in response to electrical signals generated by a microprocessor and are deposited on a print medium, such as paper or polymeric substrates, to form the desired image.
When a high-resolution image is printed, major issues arise, namely, color quality and image permanence, i.e., how long the quality of the image will last. With respect to much of the print media currently on the market, printed images commonly exhibit undesirable characteristics relating to image permanence. One such undesirable characteristic is the gradual dye-fade that is sometimes observed when dye-based ink-jet inks are printed on porous media. Such fade is caused by air and, more particularly, by small amounts of ozone in the air. It appears that, over time, ozone reacts with many dyes commonly used in ink-jet inks, causing them to break down and to lose or diminish their intended color properties. It has been observed that when ozone reacts with ink-jet ink dyes, the intended color properties of a given dye may shift to another wavelength value along the visible spectrum. This effect causes a gradual change in the perceived colors of the printed image from what was originally intended, i.e., colorshift. For instance, “red shift” is caused by oxidation of cyan dyes. Existing techniques for mitigating red shift include protecting prints from exposure to the atmosphere, such as by displaying them under glass. Dye-fade and colorshift is more of a problem with certain dyes than with others. Cyan dyes tend to be affected to a greater extent by the presence of ozone in the air than do other dyes.
Some of the drawbacks associated with the use of existing ink-jet inks on conventional print media include bronzing on swellable photo media, bleeding of colors, and stacked haze on porous photo media, among other difficulties. “Bronzing” refers to a lustrous sheen that appears on a printed sample in reflected light, typically appearing as reddish-brown color. It tends to be associated with cyan dyes. Other chemical reactions which detract from the desired color quality can take place in the ink composition during storage. An ink-jet ink composition should be stable and capable of producing the desired color after several months in storage, and for some period of time after first use of the ink and exposure to air.
“Humid hue shift” refers to the tendency of the ink and/or the colorant(s) present in the ink to migrate or spread undesirably into unprinted or printed adjacent areas of the media substrate. Humid hue shift typically occurs prior to the printed inks fully drying on a substrate or after the inks are printed when exposed to humid conditions.
“Image quality” generally refers to the level of defects including, but not limited to, bronzing, humid hue shift and/or bleed of ink-jet printed marks, particularly with colored inks.
Black-to-color bleed is a well recognized problem in the industry. One of the issues confronting formulators of inkjet ink sets comprising black and color (e.g., cyan, yellow, magenta) is that of black-to-color bleed. This has been an issue earlier for dye-based colorants and, more recently, for pigment-based colorants, and most recently for self-dispersed pigment-based colorants.
The term “bleed” as used herein, is defined as follows: When inks of two different colors are printed next to each other, it is desired that the border between the two colors be free from the invasion of one color into the other. When one color does invade into the other, the border between the two colors becomes ragged, and this is bleed. This is in contradistinction to uses of the term in the prior art, which often defines “bleed” in the context of ink of a single color following the fibers of the paper.
The term “halo” applies to the printing defect that occurs in black areas surrounded by color ink. In some cases, a distinct light-gray zone is seen where the black is adjacent to color. The defect is normally seen within 2 to 3 mm from the line dividing the black and color areas and occurs on the black side.
Finally, “mottle” is defined as a color or black non-uniformity on top of the paper, with some areas darker than others.
Dye-based systems, in which the dye colorant is soluble in water, have different considerations than pigment-based systems, in which the pigment colorant is insoluble in water and must be rendered dispersible, such as by the use of polymeric dispersants or by self-dispersing the pigment.
Adding polymers to pigmented systems is well known in the art. Polymers tend to stabilize the dispersion, as well as improve such attributes as the black-to-color bleed, optical density of the black, halo, and mottle. For example, pigmented inks have been disclosed that contain AB and BAB block-copolymers, where the blocks are made out of various monomers of acrylic family. As another example, a relatively high molecular weight (>10,000) polysaccharide such as alginic acid or carrageenan has been used. As another example, the use of polymeric resins, in particular, styrene-acrylic copolymers in pigmented inks is known. Still further, inks containing a carboxylic salt polymer are known. Also, inks containing a copolymer of a hydrophobic α,β-unsaturated ethylene monomer, and a hydrophilic monomer having a plurality of COOM groups (where M is hydrogen, alkali metal, or an ammonium ion) are known. Specifically, the hydrophobic monomer can be styrene and hydrophilic monomer can be maleic anhydride. In addition, an ink-jet ink containing two dispersants: one containing a sulfonic acid salt/ester, and the other containing a carboxylic acid salt, within a specific range of molecular weights has been described. Finally, a number of polymers for the bleed and halo control are described, most of them being of the acrylic type.
It has been known in the art that introducing chemical reactivity between the color and black inks brings many positive features to ink-jet performance, such as color-to-black bleed control and decreasing the halo. Also, it has been discovered that if reactive color inks are used for underprinting under the black areas, the optical density of the black is improved. The interaction between the polymeric dispersant and polyvalent metal ions in the color inks has been described for controlling bleed/halo/mottle. Alternatively, the reaction with the polymer can be triggered by organic acids in the color inks.
Aside from polymeric dispersants, there is another approach for preparing colloidal pigment dispersions, called self-dispersion. It typically involves attaching solubilizing groups to the pigment. There are several methods known in the art that enable the attachment of a solubilizing ionic group to the surface of a pigment particle. For example, a method of attaching an ionic group to the carbon surface is based on the reactions of diazonium salts. On the other hand, a reaction by oxidation of the surface of the pigment particles may be performed using ozone.
Self-dispersed pigments are much more difficult to control. Developing black-to-color bleed control mechanisms for self-dispersed black pigment inks, such as those available from Cabot Corp., is typically harder than for conventionally dispersed black inks (e.g., using polymeric pigment dispersants) because the self-dispersed pigments are highly electrostatically stabilized. In addition, there is no polymeric dispersant which, when reacted, rapidly builds viscosity and limits ink migration. As an example, a series of polymers of the acrylic type have been used to improve the color-to-bleed and halo of the black pigmented inks. Further improvements are needed for black-to-color-bleed control, especially for self-dispersed black pigment inks.
Bleed Control Agents
The state of the art approach to this black-to-color bleed problem is to add multivalent salts to the dye based color inks that will be printed with the black pigmented inks. The styrene/maleic anhydride polymers in the black ink interact with incompatible multivalent (inorganic or organic) salts in the color, ink. These multivalent salts must be soluble in the ink in the concentration employed. Salts customarily employed in such color inks include nitrate, chloride, acetate, perclorate, formate and thiocyanate salts of calcium and magnesium. The use of salts to control black-to-color bleed in color inks is discussed in U.S. Pat. No. 5,536,306.
One of the issues with respect to the current state-of-the-art approach of modifying dye-based color ink formulations by addition of salts for black-to-color bleed control, is the potential irritancy of the ink. Therefore, a need remains for further improvement of the black to color bleed control using salts that are more environmentally friendly. Ink-jet inks that are more environmentally friendly or “green” would be welcomed by the industry.